Castor A is one of six stars that make up Castor in Gemini: three pairs bound together, a system that helped show that gravity rules beyond the Sun.
At a glance
| type | white star (class A) |
|---|---|
| distance from the Sun | 50.9 light-years (15.6 pc) |
| its light left it | 50.9 years ago |
| luminosity | 37 × the Sun |
| radius | 2.4 × the Sun |
| surface temperature | 9,500 K |
| constellation | Gemini (Twins) |
Read off the piece’s own catalogue, the same values it draws with.
About Castor A
Through a telescope Castor splits into a bright double, Castor A and Castor B, both hot white stars. Each of those is itself a close pair, revealed only by the regular shifts in their spectra. A third pair of red dwarfs, Castor C, eclipses itself as it orbits and is bound to the rest at a wider distance.
Castor was among the doubles that William Herschel watched change position over the years. He concluded that such pairs are not chance alignments but stars in orbit around each other, an early demonstration that gravity works between the stars as it does in the Solar System.
What you see in the piece
With HUMAN EYE on, Castor and Pollux are the twin heads of Gemini; Castor is the whiter of the two. Fly out and Castor falls behind Pollux, the twins lying at quite different distances.
Questions
How many stars are in Castor?
Six: three pairs, Castor A, Castor B and Castor C, all gravitationally bound together.